[WIP] Fix timezone round-trip bug in BackfillWorkflowExecutorDelegate

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Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

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Background

PR apache#18003 (apache/dolphinscheduler) implements downstream-dependent backfill triggering in BackfillWorkflowExecutorDelegate. There are two remaining issues that must be fixed without introducing any new problems:

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Issue 1 — Timezone round-trip bug (functional bug, highest priority)

File: dolphinscheduler-api/src/main/java/org/apache/dolphinscheduler/api/executor/workflow/BackfillWorkflowExecutorDelegate.java

Root cause: doBackfillDependentWorkflow currently accepts List<String> backfillTimeList (date strings), then converts them back to ZonedDateTime via DateUtils.stringToZoneDateTime. This creates a string→ZonedDateTime round-trip. Because stringToZoneDateTime parses using ZoneId.systemDefault(), the timezone can shift when the request timezone differs from the JVM default, causing downstream workflows to be backfilled for wrong business dates.

Fix:

• Change the signature of doBackfillDependentWorkflow to accept List<ZonedDateTime> backfillDateList instead of List<String> backfillTimeList
• Pass the ZonedDateTime list directly to the downstream BackfillParamsDTO.backfillDateList, eliminating the string→ZonedDateTime conversion entirely
• Update all call sites: in doBackfillWorkflow, instead of passing backfillTimeList (strings), pass the original ZonedDateTime list. The ZonedDateTime list is available from backfillWorkflowDTO.getBackfillParams().getBackfillDateList() — but note that in the serial path the list has already been sorted (in doSerialBackfillWorkflow), and in the parallel path each chunk is a sub-list of ZonedDateTime. So the correct approach is to pass the ZonedDateTime chunk/list into doBackfillWorkflow, and then pass it along to doBackfillDependentWorkflow.

Concretely:

1. Change doBackfillWorkflow to also accept List<ZonedDateTime> backfillDateList alongside List<String> backfillTimeList — OR store the ZonedDateTime list on the DTO so it is available in doBackfillWorkflow. The cleanest approach: change doBackfillWorkflow to receive both List<ZonedDateTime> backfillDateList and List<String> backfillTimeList (the string list is still needed for the trigger request to master), and pass backfillDateList through to doBackfillDependentWorkflow.
2. In doSerialBackfillWorkflow: pass backfillTimeList (the sorted ZonedDateTime list) plus the string conversion to doBackfillWorkflow.
3. In doParallelBackfillWorkflow: pass stringDate (the ZonedDateTime chunk) plus the string conversion to doBackfillWorkflow.
4. In doBackfillDependentWorkflow: remove the DateUtils.stringToZoneDateTime conversion; use the passed-in ZonedDateTime list directly as backfillDateList in dependentParams.
5. Remove the now-unused DateUtils import if it is no longer needed (check — DateUtils::dateToString is still used, so keep the import).

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Issue 2 — Reflection-based test access (brittle tests)

File: dolphinscheduler-api/src/test/java/org/apache/dolphinscheduler/api/executor/workflow/BackfillWorkflowExecutorDelegateTest.java

Root cause: All tests that exercise doBackfillDependentWorkflow use getDeclaredMethod + setAccessible(true) to call the private method. This is brittle: any rename or signature change silently breaks tests.

Fix:

• Change doBackfillDependentWorkflow in the production class from private to package-private (no modifier) — it is in the same package as the test
• Remove ALL reflection usage from the test: replace every getDeclaredMethod / setAccessible / method.invoke(...) block with a direct call to backfillWorkflowExecutorDelegate.doBackfillDependentWorkflow(...)
• Remove the import java.lang.reflect.Method; import from the test file since it will no longer be needed

The method signature after the timezone fix (Issue 1) will be:

void doBackfillDependentWorkflow(BackfillWorkflowDTO backfillWorkflowDTO,
                                  List<ZonedDateTime> backfillDateList,
                                  Set<Long> visitedCodes)

Tests must be updated to pass List<ZonedDateTime> instead of List<String> when calling doBackfillDependentWorkflow.

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Issue 3 — PR description not updated

File: PR body (not a source file — skip this, it is handled separately)

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Constraints

• Do NOT change the public API surface (execute, executeWithVisitedCodes, triggerBackfillWorkflow)
• Do NOT change existing test logic/assertions — only change how the private method is accessed (direct call vs reflection) and update the argument type from List<String> to List<ZonedDateTime>
• Do NOT introduce any new bugs
• Keep the processService and commandDao fields as-is (they exist in the class, don't remove them)
• The DateUtils import must remain (still used for DateUtils::dateToString)
• LinkedHashSet import may still be needed (used in doBackfillDependentWorkflow for downstreamCodes)
• Ensure the test...

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